1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, to a reduced pressure drop debris filter bottom nozzle for a fuel assembly of a nuclear reactor.
2. Description of the Related Art
During manufacture and subsequent installation and repair of components comprising a nuclear reactor coolant circulation system, diligent effort is made to help assure removal of all debris from the reactor vessel and its associated systems which circulate coolant therethrough under various operating conditions. Although elaborate procedures are carried out to help assure debris removal, experience shows that in spite of the safeguards used to effect such removal, some chips and metal particles still remain hidden in the systems. Most of the debris consists of metal turnings which were probably left in the primary system after steam generator repair or replacement.
In particular, fuel assembly damage due to debris trapped at the lowermost grid has been noted in reactors. Debris enters through the fuel assembly bottom nozzle flow holes from the coolant flow openings in the lower core support plate when the plant is started up. The debris tends to become lodged in the support grids of the fuel assembly within the spaces between the xe2x80x9cegg-cratexe2x80x9d shaped cell walls of the grid and the fuel rod tubes. The damage consists of fuel rod tube perforations caused by fretting of debris in contact with the exterior of the tube. Flowing coolant causes the debris to gyrate which tends to cut through the cladding of the fuel rods
Several different approaches have been proposed and tried for carrying out the removal of debris from nuclear reactors. One approach has been to form a large number of nozzle holes in the plate member of the bottom nozzle, the holes being smaller than the maximum dimension across flow passages through the grids downstream of the nozzle. Such debris filter bottom nozzles trap debris large enough to become lodged in the downstream grids. Debris small enough to pass through the debris filter bottom nozzle continues through the passages of the grids without becoming lodged in the grids or causing fuel rod damage.
The debris filtering effect of the bottom nozzle has been enhanced by the lowermost grid. The lowermost or protective grid is similar in design to other fuel assembly structural grids, with interlocking straps forming square cells around each fuel rod and thimble tube. The protective grid is located at the bottom of the fuel assembly closely adjacent the bottom nozzle. The flow holes in the bottom nozzle are aligned with the protective grid straps such that the cross sectional centers of the holes are either axially aligned with an intersection between a pair of interlocking straps or are centered at the midpoint of a strap at the side of a cell. In such manner, the holes that are centered on strap intersections are quartered and those aligned with strap midpoints are bisected. The maximum debris size that can pass through such a bottom nozzle/protective grid combination is accordingly reduced.
While such debris filter bottom nozzles have been effective for their intended purposes, such nozzles have not been without limitation. For instance, it is desirable to reduce the pressure drop of the water flowing through the holes of the bottom nozzle. As such, it is desired to provide an improved debris filter bottom nozzle that has a reduced pressure drop without meaningfully impairing the debris filtering effect of the bottom nozzle.
In view of the foregoing, a reduced pressure drop debris filter bottom nozzle for use in a fuel assembly of a nuclear reactor includes a plate member formed with a plurality of first flow holes and a plurality of second flow holes. The first flow holes are oblong in cross section and may be of an oval configuration. The first flow holes each have in cross section a major axis and a minor axis, the major axis being longer than the minor axis. The second flow holes are circular in cross section and may have a diameter substantially equal to the minor axis of the first flow holes. The bottom nozzle also features support ligaments that extend between the first flow holes and that are axially aligned with fuel rods within the fuel assembly.
An aspect of the present invention is to provide a bottom nozzle for a fuel assembly of a nuclear reactor having a plurality of first flow holes that are oblong in cross section.
Another aspect of the present invention is to provide a debris filter bottom nozzle for a fuel assembly of a nuclear reactor having a reduced pressure drop across the nozzle.
Another aspect of the present invention is to provide a debris filter bottom nozzle for a fuel assembly of a nuclear reactor in which the nozzle is formed with a plurality of flow holes that, when cooperating with a protective grid of the fuel assembly, resist the passage of debris that would be sufficiently large to become lodged in the passages of the grids of the fuel assembly.
Another aspect of the present invention is to provide a fuel assembly for use in a nuclear reactor having a debris filter bottom nozzle with flow holes configured thereon that reduce the pressure drop across the bottom nozzle yet still resist the passage of debris into the fuel cell that is large enough to otherwise become lodged in the fuel cell.
Another aspect of the present invention is to provide a debris filtering nozzle for use in a fuel assembly of a nuclear reactor of the type having a grid and a plurality of fuel rods, in which the grid includes a plurality of first straps aligned with one another and a plurality of second straps aligned with one another, and in which the general nature of the nozzle can be stated as including a plate member having a substantially planar first surface and a substantially planar second surface opposite one another, the plate member being formed with a plurality of first flow holes and a plurality of second flow holes extending between the first and second surfaces, the plurality of first flow holes being oblong in cross section, the first surface being structured to be disposed adjacent the grid.
Another aspect of the present invention is to provide a fuel assembly of a nuclear reactor, in which the general nature of the fuel assembly can be stated as including a plurality of fuel rods, a grid having a plurality of first straps aligned with one another and a plurality of second straps aligned with one another, the first and second straps together defining a plurality of cells in the grid, the fuel rods being carried in the cells, a debris filtering nozzle including a plate member having a substantially planar first surface and a substantially planar second surface opposite one another, the first surface being disposed adjacent the grid, the plate member being formed with a plurality of first flow holes and a plurality of second flow holes extending between the first and second surfaces, the plurality of first flow holes being oblong in cross section, and at least one of the first and second straps extending across at least one of the first flow holes.